1. Field of the Invention
The present invention relates to a method for working an article that is composed, at least in part, of cubic boron nitride (hereinafter referred to as "a cubic boron nitride article"), which can be applied to a tool, a heat sink, and the like. In particular, the present invention provides a method for effectively and accurately cutting and etching a cubic boron nitride article and smoothing a surface of a cubic boron nitride article.
2. Description of Related Art
Cubic boron nitride articles are generally known for displaying high degrees of hardness. This attribute makes cubic boron nitride articles extremely difficult to work. For instance, where cubic boron nitride is grown of a single crystal, the article thereof can be conventionally worked by an abrasion grinding method. Such a conventional method only provides limited results, since abrasion grinding can only be performed along the plane orientation (or plane direction) of the single crystal cubic boron nitride article. By contrast, where cubic boron nitride is grown of a polycrystal, smoothing a surface of the cubic boron nitride article is extremely difficult, since the plane orientation of each grain which constitutes the polycrystal is arbitrary.
If the cubic boron nitride article is produced by a sintering method wherein a conductive sintering aid is used, a conventional electrical discharge method can be performed to cut the article. However, the electrical discharge method displays several limitations in terms of its applicability. For example, the electric discharge method can not be applied to a cubic boron article grown of a single crystal or to a non-conductive cubic boron nitride article, since conductive sintering aids are absent from such articles.
To overcome this problem, a conventional thermal cutting method incorporating a laser beam (e.g., CO.sub.2, CO, or YAG lasers) has been used. Unfortunately, cubic boron nitride is transparent to each of these laser beams. Thus, utilization of such laser beams produces disadvantageous and ineffective results.
Heating and cutting a cubic boron nitride article with an infrared ray having a wavelength above 1 .mu.m generates a hexagonal crystal phase, thereby disturbing the crystallinity of the article. As a result, the hexagonal crystal phase can altogether prevent the article from being cut. Alternatively, attempts to cut a cubic boron nitride article having a hexagonal phase can cause contamination at the cut surface. In addition, an area in the vicinity of the cut region is degraded by heat from the infrared ray. Further, because a relatively wide cutting margin is required, working accuracy is impeded.
Similarly, cutting a cubic boron nitride article by means of a conventional laser beam produces a high degree of heat, which degrades the cubic crystal structure around the vicinity of the cut region. The degraded region can generally be mechanically scraped off by, for example, an abrasion method. Smoothing a cubic boron nitride article by means of a conventional laser beam also results in a disadvantageous thermal degradation of the cubic crystal structure around the vicinity of the cut region. In addition, when attempting to smooth a cubic boron nitride article by a conventional laser beam, the laser beam often reaches an interior or base portion of the article, producing further adverse consequences. To overcome these disadvantages, an angle of incidence of the laser beam should be limited so as to only irradiate a surface layer of the cubic boron nitride article. Alternatively, a means may be incorporated for suppressing the scattering of the laser beam at the surface of the article.
Abrasion treatment often affects the cubic boron nitride article in an undesirable manner. Grinding the cubic boron nitride article by an abrasion technique can heat the article to several hundred degrees centigrade (e.g., 900.degree. C.). However, it is known that cubic boron nitride structure begins to transform to the hexagonal crystal at temperatures above one thousand degrees centigrade. In addition, abrasion treatment can make the fixing of the cubic boron nitride difficult. In particular, cubic boron nitride articles possessing diminutive sizes or irregular shapes are extremely difficult to work. In some cases, it is impossible to work the article by the abrasion technique. In addition, abrasion treatment often requires the expenditure of a large amount of time.